The line ups were a bit unusual, to say the least. On one side, you have some thousands of Communist party workers (probably not well armed) loosely aligned with several hundred thousand workers. On the other side, you have elite Nationalist army units, some 5,000 of the best trained and equipped elite troops in all of China, under General Bai Chong Xi plus gangs such as the so-called "Green Gang" led by Du Yue Sheng.

The four groups initially worked together to defeat the warlords holding Shanghai. The Chiang's troops and the Green Gang together turned on the Communists and workers on April 12, 1927.

Why did Chiang feel the need to enlist the Green Gang in these efforts? Couldn't his Army troops do the job by themselves? If not, what did the Green Gang add to tip the balance of power in Chiang's favor?

Or did Chiang Kai Shek feel the need to work with the Green Gang because he feared that his Army troops would not be able to counter a "three way" coalition of Communists, workers, and the Green Gang itself?

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For what I remember, Chiang's relationship with the Green Gang is addressed in detail in Sterling Seagrave's The Soong Dynasty ("One loved power, one loved money, one loved China: three remarkable sisters who shaped China's greatness") and to a lesser extent also in Jay Taylor's The Generalissimo: Chiang Kai-shek and the Struggle for Modern China. Here's from the introduction of a Seagrave's 1985 account:

Chiang's direct connection with the notorious Shanghai Green Gang
after the winter of 1926-27 has been known for many years, but there
has been only a vague understanding that those links went back much
earlier, and of how they affected this career. It is now possible for
the first time to see the "Divine Skein" linking them all the way back
to his youth, before 1910, and the manner in which the Green Gang
leaders used Chiang decisively (and were used by him) to snatch the
revolution from the hands of Dr. Sun Yat-sen's coalition. Things that
have only been rumoured can now be seen rather starkly, and Chiang's
takeover, which never made much sense in the official version, begins
to make sense now for the first time.

Unfortunately at present I cannot remember how events unfolded in this book's account, so I'd rather direct you to it rather than revisiting it myself all-too-quickly.

BTW, I do remember repeating fun occurrences of Chiang's dentures in Taylor's biography of the great man :)

The Green Gang were among Chaing Kai-shek's largest financial backers - they saw the communists as bad for business, and the Nationalist forces welcomed the money and street-level intel the triad could provide. Kai-skek needed the Green Gang to retain his hold on the KMT over challenges from Jingwei, and bring the left wing of the party to heel.

I am writing this answer as the son of two Chinese immigrants to the U.S. My father happens to be a nephew of this man: http://en.wikipedia.org/wiki/Liang_Shiyi. Elder members of my mother's family were also involved in Chinese politics. As my mother would say, "Chiang Kaishek did not have a very big chest" [heart]

Chiang's army could handle a few thousand Communists by itself, no problem. The issue was that Chiang was also against the labor unions, whose members numbered in the hundreds of thousands. In this particular fight, he didn't even have the full support of his own (Nationalist) party. So he had to user "outsiders" who happened to be his personal friends.

The Green Gang provided more than just "guns." They were "locals" to Shanghai, which means that they could find political opponents more easily than Chiang's army. Also, they did a lot of "dirty," extralegal work.

According to this link http://en.wikipedia.org/wiki/Shanghai_massacre_of_1927 some two or three hundred Communists (representing about 1% of a Communist party numbering in the tens of thousands) were arrested, and executed with great fanfare. But there were 5,000 "missing," mostly labor unionists, representing about 1% of the union members. Most of these died "accidentally on purpose' ("jumped" out of a tall building, "fell" into the river, suffocated and entombed in some cellar, etc.) Interestingly, top level Communists like Chen Duxiu and Zhou Enlai were not killed, because Chiang's strategy was spare the generals and slaugther the captains (the 1% below the very top).